Geological Framework

Geology of onshore suriname

Geologically, Suriname onshore is subdivided into a crystalline basement (80%), and a coastal plain (20%).

The crystalline basement forms part of the Guiana Shield, which stretches between the Orinoco and the Amazon rivers and includes eastern Venezuela, Guyana, Suriname, French Guiana, and northern Brazil. The crystalline basement is formed principally of igneous and metamorphic rocks, whilst the coastal plain, which stretches along the northern fringe of the shield area, is exclusively sedimentary. The offshore extension of the coastal plain is a large sedimentary basin that was formed mid to late Jurassic and is filled with clastic and carbonate sediments.

Stratigraphy

The stratigraphy of the Guyana-Suriname Basin reflects two-phases of geodynamic evolution.

Phase one, the Central Atlantic (CA) phase can be subdivided into two mega-sequences corresponding to a rift-to-drift history. This rift phase, mid to late Jurassic, is equated to a syn-rift sequence but has only been penetrated by one well (A2-1) offshore Suriname. The Central Atlantic drift mega-sequence relates to early Cretaceous deposits comprising moderate to high fluvial/deltaic discharge, rapidly deposited in an inner Neritic environment. The end of this period is marked by the Mid Albian unconformity, also called the ‘Break-up’ Unconformity, as it marks the beginning of the Equatorial Atlantic drift apart of Africa and South America.

Phase two, the Equatorial-Atlantic (EA) phase has a drift/passive margin history. The Equatorial Atlantic Drift mega-sequence developed as a monoclinal wedge of sediments which built out over the continental shelf from late Cretaceous to present day. Several transgressive-regressive cycles can be recognized, but the sequence is overall progradational in nature from the Turonian-Coniacian to the present day, presenting a general basin-ward advance of the shelf-edge. Several unconformities, formed due to fluctuating sea level and episodes of basin margin uplifts, are recognized.

Basin Architecture

Major structural elements of the Guyana-Suriname Basin include the Jurassic syn-rift grabens underlying the shelf, the folded and faulted late Jurassic and early Cretaceous sediments that underlies the late Cretaceous and Tertiary deltaic monocline which was not subjected to a significant tectonic event. The third recognized structural element is the Demerara Plateau which demarks the eastern extent of the basin. The plateau is a remnant of older rocks from Gondwanaland pre dating its breakup into Africa and South America. Folded and faulted early Jurassic and early Cretaceous rocks are overlain by a somewhat thinner, un-deformed section of late Cretaceous to Tertiary rocks.

The major expression of Lower Cretaceous compression is the folded and faulted Demerara Plateau, a scarcely explored, old (Jurassic-Lower Cretaceous) basin. Following the uplift, large volumes of Early Cretaceous sediments were eroded to form a regionally extensive erosion surface, the Aptian/ Albian unconformity.

The main architectural elements of the basin are illustrated in this regional composite seismic section, which stretches from the shelf to the Demerara Plateau. Note the length of the profile as well as the time scale.

Petroleum Geology

The presence of hydrocarbons within the basin was proven by the discovery of >1 Billion barrels STOIIP in the Tambaredjo, Calcutta and Tambaredjo NW Fields onshore Suriname, and by the presence of oil and gas shows and oil columns in offshore wells.

The most significant source rock developed in the Guyana-Suriname Basin is the Canje Formation that ranges Upper Albian to Santonian. These rocks have been encountered in several wells offshore Suriname and Guyana. This is a regionally recognised source rock interval with high TOC being time-equivalent to the prolific La Luna Formation of Venezuela and Naparima Hill in Trinidad.

Analysis of hydrocarbons from the Cretaceous interval proved that there is a second source rock. The oil tested was generated from a lacustrine Jurassic Source. There is further evidence of the presence of a Jurassic source in the analysis of oils produced from several wells drilled onshore Suriname in Coesewijne, Weg naar Zee and Commewijne. These oils were interpreted to have derived from an unknown source rock, probably strongly restricted, lacustrine environment with characteristic Jurassic age biomarkers.